The twenty-step synthesis of cholesterol from lanosterol is catalyzed by microsomal enzymes. The sequence of enzymic steps is being elucidated by interrupting the biosynthetic process catalyzed by the multienzymic system. Then, each enzyme or group or enzymes is isolated and purified. Segments of the biosynthetic system are being reconstituted by combination of the purified proteins and phospholipids. To date, the sequence of the central ten steps has been elucidated, and most of the proteins that catalyze this half of the twenty-step process have been isolated and purified. Concurrently, regulation of the rates of these enzymic transformations and the relationship of microsomal oxidases of cholesterol biosynthesis to other microsomal electron transport reactions are under investigation. Current evidence indicates that 4-methyl sterol oxidase of liver microsomes is the rate-limiting reaction of cholesterol synthesis from lanosterol. BIBLIOGRAPHIC REFERENCES: Gaylor, J.L., Miyake, Y., and Yamano, T. (1975). Stoichiometry of 4-methyl sterol oxidase of rat liver microsomes. J. Biol. Chem. 250: (no. 18, Sept. 25 issue). Williams, M.T., Gaylor, J.L., and Morris, H.P. (1975). Microsomal enzymes of cholesterol synthesis in Morris hepatomas. Fed. Proc. 33: 1925 (abs).